The present invention relates to an image display apparatus, such as a liquid crystal projector, for projecting a light beam supplied from a light source via an image display element on a screen as a magnified image.
In a liquid crystal display (LCD) device having pixel electrodes arranged orderly in a matrix, driving voltages corresponding to image signals are applied to the respective pixel electrodes, to change the optical characteristics of a liquid crystal material sealed in the LCD device. In this way, the LCD device displays images, characters, and the like. The application of individual driving voltages to the pixel electrodes is realized by a simple matrix method or an active matrix method. In the latter method, the LCD device is provided with nonlinear two-terminal elements or three-terminal elements.
In the active matrix method, required are switching elements such as MIM (metal-insulator-metal) elements and thin film transistors (TFTs), as well as wiring electrodes for supplying driving voltages to the pixel electrodes.
Such an active matrix LCD device has drawbacks as follows. When intensive light is incident on the switching element, the resistance of the switching element in the OFF state lowers, resulting in discharging a charge accumulated during voltage application. Moreover, light leaks in the black state from the portions of the liquid crystal material corresponding to the regions where the switching elements and the wiring electrodes are formed, because such liquid crystal portions are not applied with a normal driving voltage and thus fail to participate in the display operation done by the other portions. This results in lowering the contrast ratio.
To overcome the above drawbacks, a transmission LCD device is required to provide a light-shading section called a black matrix 602 as shown in FIG. 31 for shading the above regions from incident light. Referring to FIG. 31, the black matrix 602 is formed on a counter substrate that faces via a liquid crystal layer a TFT substrate on which the switching elements such as TFTs 601 and pixel electrodes 605 are formed. The TFTs 601 as well as gate bus lines 603 and source bus lines 604 intrinsically have the light-shading property. Therefore, in the case of the transmission LCD device, the formation of the light-shading black matrix 602 further reduces the occupation of an effective pixel aperture in the entire area of a pixel, that is, the numerical aperture.
It is difficult to reduce the sizes of the switching elements and wiring electrodes below a certain level due to the restrictions in the electrical performance and production technology. Therefore, as the pitch of the pixel electrodes is made smaller in the course of implementation of more precise, smaller LCD devices, the numerical aperture is further reduced.
A reflection LCD device has been developed to solve the above problem.
In a reflection LCD device, as shown in FIG. 32, reflection type pixel electrodes 655 can be formed over TFTs 651 as the switching elements, thereby allowing for a larger numerical aperture than that of the transmission LCD device described above. The reflection LCD device is therefore very effective in improving the brightness of a projection LCD apparatus.
Applications of such a reflection LCD device to a projection LCD apparatus are proposed in Electronic display Forum 97 (pp. 3-27 to 3-32) and Japanese Laid-Open Patent Publication No. 4-338721.
The proposal in the Electronic Display Forum 97 is as shown in FIG. 33. A light beam emitted from a light source 701 is separated into light beams of the three primary colors of red (R), green (G), and blue (B) by dichroic mirrors. The three light beams are guided to be incident on corresponding polarization beam splitters (PBS) 702. The PBS 702 splits the incident light beam into two linearly polarized light components in directions orthogonal to each other. One of the light components is incident on a corresponding reflection LCD element 704. After being reflected from the reflection LCD elements 704, the R, G, and B light beams with a modulated polarization direction are incident again on the respective PBSs 702. The light beams are then combined by a cross dichroic mirror 703, and the combined light beam is projected on a screen via a projection lens 705.
The proposal of Japanese Laid-Open Patent Publication No. 4-338721 is as shown in FIGS. 34A and 34B. A light beam emitted from a light source 101 is split into two linearly polarized light beams by a PBS 105. One of the two light beams is separated into R, G, and B light beams by a color separation/combination element (a cross dichroic prism in FIG. 34A, and a Phillips type prism in FIG. 34B). After being reflected from respective reflection LCD elements 107-R, 107-G, and 107-B, the color light beams are combined by the color separation/combination element. The combined light beam is incident again on the PBS 105, where only the light components of which polarization direction has been modulated are incident on a projection lens 108 and projected on a screen.
The above method is implemented as a 3-panel type liquid crystal projector, which allows efficient use of the R, G, and B light beams from the light source and thus display of markedly bright images.
The 3-panel type liquid crystal projector uses all the R, G, and B light beams for display at all times (spatial color mixing). Another method called a time sequential method or a field sequential method is known, where a light beam for display is time-divided into the R, G, and B light beams to effect color display.
For example, in a liquid crystal projector disclosed in Japanese Laid-Open Patent Publication No. 5-158012, R, G, and B light components of a light beam emitted from a light source are sequentially selected in a time-division manner to be sequentially incident on a PBS. The PBS splits the incident light beam into a p-polarized light component and an s-polarized light component, and outputs one of the two light components (in the disclosed example, the s-polarized light component) to be incident on an image display element. The image display element receives a signal synchronized with the presently incident color light beam. Display is made for each cycle of the R, G, and B light beams as a unit (called a frame; the unit of display for each of the R, G, B light beams is called a field).
For the time-division display of the R, G, and B light beams, the disclosed liquid crystal projector uses a combination of a dichroic mirror and a shutter. The dichroic mirror separates white light emitted from the light source into the R, G, and B light beams. The shutter allows/blocks transmission of the separated color light beams. As an alteration, a rotary color filter 393 having R, G, and B transmission regions as shown in FIG. 39 may be used. The projector of this type (a single-panel type projector, named against the above 3-panel type projector) may be constructed of only one reflection image display element and only one PBS. Moreover, it requires no individual optical systems for color separation and color combination, allowing for realization of an inexpensive, compact system.
However, the above conventional apparatuses have the following problems.
In the proposal in the Electronic Display Forum 97, three PBSs for the R, G, and B colors are required, together with the optical systems for color separation and the cross dichroic prism for color combination. This greatly increases not only the cost but also the size of the system.
In the proposal in Japanese Laid-Open Patent Publication No. 4-338721, the size of the system can be reduced because color separation and combination are implemented by one element and only one PBS is required. However, the construction using the cross dichroic prism has the following problem. Normally, light is incident on a color separation plane of the cross dichroic prism at an angle of 45.degree.. This increases the polarization dependence of the spectral characteristics at the color separation plane as shown in FIG. 35 (FIG. 35 shows the spectral characteristics of B light at the color separation plane). Therefore, if both color separation and color combination are done by the cross dichroic prism, the bandwidth of light usable is greatly narrowed, resulting in poor light utilization efficiency and lowered color purity.
In the construction using the Phillips type prism, light is incident on the color separation plane at a small angle compared with that using the cross dichroic prism. Therefore, the polarization dependence as described above is reduced. The Phillips type prism however has a drawback that the optical path through the Phillips type prism is long. In order to allow incident light to pass through the Phillips type prism without an occurrence of extending off the side edges of the Phillips type prism, the prism and the projection lens must be large in size, which causes cost increase.
The time-division method disclosed in Japanese Laid-Open Patent Publication No. 5-158012 can solve the above problems of the 3-panel type projector. This is however disadvantageous in the following point. In the time-division method, the R, G, and B light beams from the light source are sequentially incident on the reflection image display element. Therefore, when the R light beam is under selection, for example, the remaining G and B light beams are substantially unusable, resulting in lowering in brightness to one third theoretically.
Moreover, assuming that one frame for image display is 1/60 second, for example, the display time allocated for each of the R, G, and B light beams is about 5 msec. Display of each color must be completed within this period of time. To satisfy this, the display element used must have a significantly high response speed. This causes a serious problem when a liquid crystal display element is used since it has a comparatively low response speed, unlike a CRT and the like.
Moreover, the time-division display of the R, G, and B primary colors tends to generate a phenomenon that the respective primary colors are observed separately from one another when moving images are displayed or when the line of sight moves (hereinafter, such phenomenon is referred to as color breaking), lowering the image quality. To reduce such color breaking, the driving frequency may be increased. To increase the driving frequency, however, the response speed of the display element must be further increased. This is also disadvantageous for the liquid crystal display element.